How make glow in Compositing if emission is behind glass?
As an alternative to blur, use a glare node. It will give you finer control since the effect is based on a threshold, to effect the picture based on the brightness of the pixels.

Read:
Any way to control light halos in the compositor based on emission level?
Also, the default glass shader will not work correctly for a light that is behind glass.
Try something like this:

Read: How to illuminate the darkness inside glass objects in Cycles?

If you must use glow, you can use a pass index to determine what gets blurred.

For complete instructions on how to set this up read:
One part of the render layer glowing?